The number of users using existing mature mobile telecommunications systems such as GSM continues to grow. In order to continue improving the spectral efficiency of GSM/EDGE Global (System for Mobile Communication (GSM) Enhanced Data for GSM Evolution), there is a need to find effective ways of multiplexing multiple users' signals into a single time slot over a 200 kHz channel while avoiding or minimizing mutual interference among users.
In accordance with a proposal in 3GPP/GERAN (third generation partnership project/GSM EDGE Radio Access Network) a fast feedback channel for Voice over IP is proposed, see GERAN#44 GP-091988 “Fast Feedback Channel” (v1). The idea is to allow the Voice over internet protocol (IP) users to signal the network when there are packets available for transmission. This channel must allow fast feedback and must consume as little bandwidth as possible. To this end, up to four users are multiplexed into one timeslot by means of time division multiplexing.
In machine-type communications there may be a need for feeding back small amount of information in the uplink for thousands of machines. Therefore, it may be desirable to multiplex multiple logical feedback channels for different machines into a single GSM time slot.
In Voice services over Adaptive Multi-user channels on One Slot (VAMOS), a standardized 3GPP/GSM feature, up to two full rate users are multiplexed in a single time slot. However, in the uplink, the receiver requires complex multi-user detection since the two user signals are ordinary Gaussian minimum shift keying (GMSK) co-channel interferers.
Accurate channel estimation and synchronization are important steps in coherent wireless technologies such as GSM/EDGE or Long term evolution (LTE). The existing channel estimation and synchronization methods in GSM/EDGE, in the case of multi-user transmission (such as VAMOS), rely on orthogonal training sequences. However, the time modulated training sequences that are employed in these methods are typically not perfectly orthogonal, which can result in significant performance losses. Moreover, in practice it is very difficult to increase the number of users sharing the same radio resource without requiring also an increase in the number of antennas at the receiver. As a result, for many cases with the current GSM/EDGE technology, it is not practical to allow more than two users on the same radio resource when the receiver only has one antenna. In addition, channel estimation and synchronization is made even more difficult by the presence of relative timing differences among the users.
It would be desirable to multiplex more than two users (say 4 or 8 users) while keeping the number of receive antennas limited to the usual number of antennas available in most commonly deployed base stations, namely one or two antennas. This is not feasible with the current training sequence design.
Hence there exists a need for an improved transmission method for mobile telecommunications systems and in particular for systems as described above.